High-performance variable speed drives and their converters place a considerable load on the power supply lines due to current harmonics. Therefore, the use of such drives requires that their converters be equipped with expensive filter circuits in order to reduce these harmonics. It is known that harmonic components can be reduced by the use of self-commutated converters having gate turn-off power semiconductors. However, at a low pulse frequency, in particular at a pulse frequency as low as the fundamental oscillation of the AC-voltage power supply network powering the drive device, a self-commutated converter may not be able to reduce the harmonics to the desired degree. Therefore, even with this measure, the use of expensive filter circuits may be necessary. This problem occurs in particular with rolling stand drives.
W/O 92/15148 describes another option for compensating harmonics. According to this reference, residual degrees of freedom in a motor control are used for minimizing an interference index. The interference index is defiend as the sum of squares or the absolute values of the difference between setpoint current and actual current.
World Patent 92/15148 discloses another option for compensating harmonics. According to this document, residual degrees of freedom in a motor control are used for minimizing an interference index. The interference index is defined as the sum of squares or the absolute values of the difference between setpoint current and actual current.
The object of the present invention is to provide a drive system for which the harmonics injected in the power supply line are reduced. In doing so, it is desirable that the drive system be designed to be more cost effective compared to known drive systems having reduced harmonics.
This object is achieved according to the present invention through a drive system according to Claim 1, according to which the link voltage of the converter is set so that the harmonics computes the link voltage as a function of the AC-voltage power supply network voltage.
The drive system according to the present invention is particularly advantageous in a power range of 1-20 MW, advantageously between 2-10 MW or, for impact load, in a power range of 2-30 MW, advantageously between 4-20 MW.
The drive system according to the present invention has been found to be particularly advantageous in connection with three-phase motors connected in tandem, i.e., in circuits where the three-phase motor has open windings, which are supplied by converters on both sides.
The drive system according to the present invention is particularly advantageous for driving rolling stands of a rolling mill train.
The converter of the drive system according to the present invention can also be designed as an air-cooled converter.